The long-term goal of this study is to understand the molecular mechanisms that regulate transcription of the large ribosomal RNAs of the frog, Xenopus laevis. In the next five years efforts will be focused on purification and characterization of trans-acting proteins that interact with already identified DNA sequences to effect this regulation. These studies will include: 1. Purification of accessory proteins that interact with the enhancers and promoters, cloning of their genes, and determination of their mechanism of action. 2. Study of the structure and regulation of RNA polymerase I. 3. Study of the relation between ribosomal RNA transcription and growth control. 4. Study the mechanism of nucleolar assembly-disassembly in vitro. 5. Transform cells to make artificial nucleoli with drug resistant ribosomal genes. 6. Locate origin(s) of replication and study mechanism of rDNA amplification.